Method and means of circumventing cathode maintenance in electron beam devices



"Skim-H March 16, 1965 l.. T. BowERs ETAL 3,174,026

METHOD AND MEANS OF CIRCUMVENTING CATHODE NCE IN ELECTRON BEAM DEVICESMAINTENA Filed June 20, 1962 j l l l l l I ATTORNEY H R .w S S R R 0 O1W. S M 0 M M m V R D E T T R S E E B L R E H Y B nited States PatentMETHOD AND MEANS F CIRCUMVENTING CATHDE MAINTENANCE 1N ELECTRON BEAMDEVICES Lester T. Bowers, Oreland, and Herbert D. Van Sciver II,

Merion, Pa., assignors to The Budd Company, Philadelphia, Pa., acorporation of Pennsylvania Filed June 20, 1962, Ser. No. 203,931 5Claims. (Cl. 219--117) This invention relates to electron beam devicesand more particularly to an anti-oxidation cathode structure.

In recent years electron beam welding, melting, metal deposition orvaporization, and machining have become well known experimentaltechniques. In some instances such as metal cutting and welding,involving small objects, these laboratory techniques have been used forsmall specialized production processes; but technical problems haveheretofore limited widespread production use.

The principle of an electron beam tool depends upon electrons emittedfrom a heated lament in a vacuum chamber. Usually a heated tungstenemissive cathode acts as an electron source and is adjustable to veryhigh negative voltages. Surrounding the negative cathode is abeam-forming cup or negative electrode which may be held more negativethan the negative cathode. Below the electrode is a target or positiveanode which is held near ground potential, as is the workpiece, and actsas an accelerator for the cloud of electrons emitted from the negativecathode and formed by the beam-forming cup negative electrode. Thecathode, electrode and anode constitute an electron beam gun. Two commonsystems are employed in shaping and focusing the electron beam; theiirst system is commonly known as the Rogowski system. Directly belowthe anode of the gun in the Rogowski system there is usually employed afirst set of beam alignment electromagnetic coils or plates to dellectand adjust the stream of electrons being emitted from the gun. ln someelectron beam machine tools an auxiliary or secondary pair of coils orplates are placed below the alignment coils to act as a manualadjustment for detlecting the beam of electrons. Below the auxiliarypositioning coils or plates the Rogowski system employs a visual lightmicroscope concentricaliy surrounding the beam of electrons. Below thelight microscope a magnetic lens refocuses and converges the electronbeam for critical focusing. Below the magnetic focusing lens anotherpair of beam dellection electromagnetic coils similar to the beamalignment electromagnetic coils is employed to permit criticalpositioning of the electrode beam. The above-mentioned Rogowski systememploying a magnetic lens permits critical focusing in a relativelyshort electron beam distance.

A secondary system known as the Steigerwald system omits the lightmicroscope and the magnetic lens but is capable of welding by extendingthe length of travel of the electron beam approximately 50% further thanthe Rogowski system. Both above-mentioned systems have been employed toperform welding.

The great advantage achieved by electron beam welding is predicated uponthe cleanliness and lack of contaminating gases in the welding area. Itis presently necessary, when employing an electron beam machine tool, tocreate a high vacuum in the order of l0-4 or 10*5 mm, of mercury (.1 to.01 micron pressure). At these vacua the impurity level in the vacuumchamber is 0.1 to 0.01 p.p.m. It was formerly believed that such a highvacuum and high purity was necessary to insure an obstacle free path forthe electron beam and prevent emissive cathode deterioration.

3,174,026 Patented Mar. 16, 1965 lCe It has been discovered that partialvacua of 1 to 150 microns pressure are suflicient to insure that theelectron beam can be adequately maintained to perform critical focuswelding, melting, metal deposition or removal and machining for oxygenremaining in the partial vacua may be substantially removed to preventrapid deterioration of the electron emitting cathode without thenecessity of creating a high vacuum. It was further discovered that theelectron beam caused oxygen and residual gases to be released from theworkpiece, which would ordinarily accelerate the deterioration of theelectron emitting cathode.

A method of prolonging the life of the emissive cathode in electron beamdevices operating at a partial vacuum has been discovered.

Therefore, it is a general object of the present invention to provide anincandescent electrode in the vicinity of the workpiece of an electronbeam device to absorb residual gases;

It is a further object of the present invention to provide anincandescent electrode having a high affinity for oxygen, nitrogen,hydrogen, etc. in the vicinity of the workpiece but removed from theeffective electromagnetic path of the electron beam;

Another object of the present invention is to provide a method ofelectron beam welding without the necessity of a high vacuum;

Another object of the present invention is to provide an electron beamwelding device for welding metals outside the vacuum chamber;

Another object of the present invention is to provide a method ofextending the life of an electron emitting cathode in a contaminatedrelatively low vacuum atmosphere.

The above and other objects and novel features of the invention will beapparent from the following description and accompanying drawing inwhich:

FIG. l is a schematic diagram of an electron beam device illustrating apreferred embodiment of the invention;

FIG. 2 is a schematic diagram of an alternative embodiment of theinvention shown in FIG. 1 showing through-metal welding with a portableunit.

In accordance with the present invention there is provided an electronbeam device comprising an electron beam gun for emitting a stream ofelectrons in a partial vacuum, a magnetic lens for focusing the streamof electrons in said partial vacuum, an incandescent electrodephysically located between the magnetic lens and a workpiece forcollecting residual impurities in said partial vacuum and an orifice inthe side of the partial vacuum enclosure cut by and having theapproximate size and shape of said stream of electrons through which theelectron beam leaves the partial vacuum and enters the atmosphere.

Referring now to FIG. l where a vacuum beam device 10 is shownsurrounded by a vacuum box 12. Negative cathode 14 is heated toapproximately 4000 F. to provide an emissive cloud of electrons 16therefrom. The cloud of electrons 16 is shaped by the negative electrodebeam-forming cup 18. Cup 18 with a control oriiice 20 therein, iselectrically insulated from the negative cathode 14, and maintained at adirect current voltage level adjustable from 0 to 100 volts D.C.negative (shown as VB) with respect to the negative cathode. The beam ofelectrons 16 is shaped by the beam-forming cup 18 and accelerated by ahigh potential positive anode 22. Depending upon the velocity of theelectron beam desired, the Voltage V0 may be varied from approximately20,000 volts to 150,000 volts. The electrostatic eld generated by thebeam-forming cup 18 focuses the cloud of electrons 16 from the emissivesurface of the negative cathode and also limits the number of electrons(i.e. beam current) projected therefrom. A high potential positive anode22 is provided with an aperture 24 therein through which the partiallyfocused electron beam 16 passes. Below the positive anode 22 there maybe provided a pair of beam alignment electromagnetic coils or plates 26and 28 to position the electron beam 16 and prevent inadvertentdefiections which could cause damage to the electron beam device. Insome embodiments of electron beam devices a tungsten diaphragm or disk30 shaped similar to the positive anode 22 is placed adjacent to thecoils 26 and 28 to insure that there is no damage incurred before thebeam alignment electromagnetic coils can be brought into effect.Directly below the beam alignment coils 26, 28 there is provided amagnetic focusing lens 32. FIG. 1 exaggerates the diffusion of theelectron beam 18 to illustrate the principle of operation of themagnetic focusing lens 32. The D.C. potential on the magnetic focusinglens 32 may be varied over a positive potential range to insure criticalfocus of the electron beam 16. The finer the focal spot is adjusted thegreater the ratio of depth to width of penetration. Higher negativecathode voltages produce greater depth to width ratios.

Should the electron beam 16 be initiated prior t0 obtaining a highvacuum of the order of l*4 mm. of mercury, the negative cathode 14 wouldordinarily be oxidized by oxygen present in the partial vacuum. Othercontaminating gases present in the partial vacuum would combine with thecathode 14 and reduce the potential current density of the beam.

Even after a high vacuum has been obtained and the electron beam 16initiated in a relatively clean chamber, the electron beam processwhether welding, machining, or vaporizing causes some outgassing of theworkpiece 34. Outgassing of the workpiece 34 not only reduces the vacuumpressure but introduces contaminating gases into the vacuum chamber 10,and can be so great as to render the process inoperable.

In order to prevent deterioration of the negative emissive cathode 14and to prolong its life there is provided an incandescent or auxiliaryelectrode 34 shielded from the magnetic lens 32 by a heat shieldinsulator 36. In many cases the incandescent electrode 34 may be made ofmaterial identical to cathode 14, such as tungsten, molybdenum andtantalum, but better results are achieved when the incandescentelectrode 34 has a higher affinity for oxygen than the emissve cathode14. In the closed vacuum chamber 12 of FIG. 1 the cathode 34 willcornbine with oxygen and absorb residual gases. Absorption of residualgases reduces the pressure in the vacuum chamber. Elimination of freeoxygen preserves the emissive cathode 14. It was found that the electronbeam device may be operated at partial vacuum pressures hundreds oftimes lower than the high vacuum pressure heretofore proposed, thusenabling electron beam process to be performed on materials which outgasexcessively.

Heretofore, electron beam devices were limited to operation on theworkpiece with the electron beam gun inside the vacuum chamber with theworkpiece. Such a requirement is no longer necessary, for the noveldevice permits introduction of gases into the vacuum chamber.

FIG. 2 is a modified embodiment of the electron beam device shown inFIG. 1. This embodiment illustrates a portable electron beam Welder forthrough-metal welding wherein the workpiece 38 is placed outside thevacuum chamber 12. In the modified embodiment the beam passes through avery small orifice 4t) in a replaceable diaphragm or cap 42 at thetarget end of the vacuum chamber. The orifice is maintained usually ofthe order of several microns, and the chamber 12 is easily maintained atpartial vacuum stability by continuous pumping of pump P. A two inchdiffusion pump is capable of pumping seventy liters per second at *4 mm.of

CTI

mercury, thus a two inch pump is large enough to maintain vacuumstability at 10-4 mm. with an Orifice having a diameter of ten microns.

The operation of the modified electron beam device is practically thesame as before. Cathode 14 initiates a cloud of electrons 16' shaped bythe cup 18 and accelerated by the anode 22. The electron beam 16 isfocused by the magnetic lens 32' and passes through the orifice 40 inthe target 42 to impinge on the workpiece 38'. The electron beam 16inside the chamber 12 is operated as before and at similar partialpressures. Well known pressure sensitive devices should be incorporatedinside the chamber to shut down the beam in case of loss' of partialvacuum, and provision made for insulating the coils to avoid damage dueto spark over if the beam is initiated in a concentrated ionizable gas.

A preferred method of operating the modified electron beam device is toinitiate the beam 16 after a partial vacuum is obtained and after theelectrode 34 is initiated. Initial focus of the beam is made at a lowvoltage level on the target 42. When it is desired to start the electronbeam process the energy level of beam 16 is increased to cut an orifice40 through the target 42. The hot center portion of the beam 16 cutsthrough the target 42 creating a very small orifice 40 approximately thesize and shape of the beam. Not only does this insure that the orificeis very small, but the orifice 40 serves as a re-focusing device toinsure that the beam 16', as it leaves the vacuum chamber 10', will becritically focused while traveling to workpiece 38. The beam 16 as itpasses from the vacuum chamber 12 must pass through a high density gasbefore reaching the workpiece 38. Part of the electron beam is ionized,but being of high velocity and high energy content remains criticallyfocused causing a beam of electrons and high velocity ionized particlesto reach the workpiece.

It is intended that the target 42 of the electron beam device be placedclose to the workpiece in operation so that the electron beam, eventhough ionized, is not diffused, but will continue in a concentratedstream to the workpiece maintaining sufficient energy and critical`focus to perform electron beam welding and other processes.

A replaceable thin metal diaphragm 42 may be employed at the target endof the vacuum chamber 12' as a seal between the vacuum chamber 12' andthe workpiece 38 located outside the vacuum chamber. The diaphragm ortarget 42 may take the form of a ribbon valve which may be changed bysliding a new piece of ribbon over the target end for replacing orsealing off the existing orifice. A baffle 44 having an orifice topermit the beam to pass therethrough may be employed at a criticallocation to deflect gases entering through the orifice 40 to the vacuumpump P.

The incandescent electrode 34 is shown as a wound wire helix in acylindrical portion of the vacuum charnber 12'. A larger surface area ofcathode 34 is more effective, but cannot be made too large withoutproviding external cooling means such as cooling fins as shown.

An inert gas shroud may be introduced at the workpiece to insure thatthere is no contamination of the workpiece as is well known in thewelding art, and the incandescent cathode can be modified to absorb theshlielding gas.

It is apparent that electron beam devices may be successfully operatedat partial vacuum and further that an electron beam may operate on aworkpiece outside the vacuum chamber. While a preferred embodiment hasbeen illustrated showing an incandescent electrode with a high afiinityfor oxygen it is apparent that plates of active metals at reactiontemperatures (below their fusion temperatures) may also be employedinside the vacuum chamber along with incandescent cathode 34 or acontinuous stream of active gas may be released into the vacuum chamberto absorb gases without any effect on the vacuum pressure in the chamberto further reduce contamination of the emmissive cathode.

Modifications and variation of the specific embodiments described hereinmay be made without departing from the spirit of the invention which islimited only by the terms of the appended claims.

What is claimed is:

1. A method of electron beam welding at atmospheric pressures comprismge step''" evacuating a singlestage vacuum chamber of an electron beamWelder to a partial vacuum pressure of approximately -4 mm. of Hg with avacuum pump, heating an auxiliary electrode in said vacuum chamber toabsorb residual gases and oxygen in said vacuum chamber, initiating alow energy stream of electrons upon a target located at the side of saidvacum chamber, said target being located directly in the path of aworkpiece located outside of said vacuum chamber, increasing theintensity of said electron beam to a higher energy level to cut anorifice in said target and permit said electron beam to impinge upon theworkpiece located outside of said vacuum chamber, maintaining saidauxiliary electrode in said vacuum chamber active to absorb gasesentering said vacuum chamber through said orice, and maintaining saidvacuum pump active to create a partial vacuum pressure in said vacuumchamber.

2. A method of prolonging the life of an emissive cathode of an electronbeam welding device having an electron beam gun in a partially evacuatedenclosure subject to the influx of contaminating gases comprising,evacuating a single-Stage sealed vacuum chamber to a partial vacuumpressure, initiating an electron beam in said vacuum chamber, creatingan opening in said vacuum chamber with said electron beam theapproximate size of the electron beam connected to the atmospherethrough which contaminating gases enter into said vacuum chamber,exhausting the single-stage vacuum chamber with a vacuum pump at a rateequal to the entrance of said contaminating gases through said opening,passing said electron beam through said opening and into the atmosphereas a concentrated beam, and heating an electrode having a high ainityfor oxygen and said contaminating gases inside said vacuum chamber toabsorb said oxygen and said contaminating gases, said electrode beingplaced intermediate said opening and the source of said electron beam.

3. A method of providing an electron stream of energy levels capable ofperforming operations on a work-piece in the open atmosphere comprising,evacuating a sealed vacuum chamber to a partial vacuum pressure,initiating a concentrated high energy electron beam in said sealedvacuum chamber, cutting an opening to the atmosphere in said sealedvacuum chamber with said electron beam, said opening having theapproximate size and shape of said electron beam, exhausting the vacuumchamber with a vacuum pump at a rate equal to or greater than theentrance of atmospheric gases through said opening, and passing saidelectron beam through said opening and into the atmosphere as aconcentrated electron stream having an energy level capable ofperforming operations on a workpiece in the atmosphere.

4. A method of conducting an electron beam into the open `atmosphere forperforming welding operations comprising, evacuating a vacuum chambercontaining an electron beam gun to a partial vacuum level of the orderof 10-4 mm. of Hg, initiating an electron beam in said partial vacuumand concentrating said beam on the side of said vacuum chamber, cuttingan opening in said vacuum chamber by increasing the energy level of saidelectron beam, said opening forming a passageway between said vacuumchamber and said atmosphere, the diameter of said opening being of theorder of 10 microns and having the approximate size and shape of saidelectron beam, said electron stream passing through said opening andleaving said vacuum chamber, and exhausting said vacuum chamber tomaintain said partial vacuum at approximately 10-4 mm. of Hg.

5. An electron beam device for operation at partial vacuum pressurescomprising, a vacuum chamber enclosure, a vacuum pump connected to saidvacuum chamber enclosure for maintaining a partial vacuum pressureinside said enclosure, an electron beam device for creating a highvelocity directed stream of electrons, a target in the path of said highvelocity stream of electrons, and an orifice in said target cut by saidhigh velocity stream of electrons having the approximate size and shapeof the electron stream, said high velocity stream of electrons passingthrough said orice leaving said vacuum chamber enclosure providing anelectron stream of energy levels capable of performing operations onworkpieces in the atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 2,208,987 7/40Kuhne et al. 313-174 2,792,517 5/57 Holland 313--174 2,899,556 8/ 59Schopper et al.

2,989,614 6/ 61 Steigerwald.

3,082,316 3/63 Greene 219-117 FOREIGN PATENTS 723,987 2/55 GreatBritain.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner.

1. A METHOD OF ELECTRON BEAM WELDING AT ATMOSPHERIC PRESSURES COMPRISINGTHE STEPS OF EVACUATING A SINGLESTAGE VACUUM CHAMBER OF AN ELECTRON BEAMWELDER TO A PARTIAL VACUUM PRESSURE OF APPROXIMATELY 10-4 MM. OF HG WITHA VACUUM PUMP, HEATING AN AUXILIARY ELECTRODE IN SAID VACUUM CHAMBER TOABSORB RESIDUAL GASES AND OXYGEN IN SAID VACUUM CHAMBER, INITIATING ALOW ENERGY STREAM OF ELECTRONS UPON A TARGET LOCATED AT THE SIDE OF SAIDVACUUM CHAMBER, SAID TARGET BEING LOCATED DIRECTLY IN THE PATH OF AWORKPIECE LOCATED OUTSIDE OF SAID VACUUM CHAMBER, INCREASING THEINTENSITY OF SAID ELECTRON BEAM TO A HIGHER ENERGY LEVEL TO CUT ANORIFICE IN SAID TARGET AND PERMIT SAID ELECTRON BEAM TO IMPINGE UPON THEWORKPIECE LOCATED OUTSIDE OF SAID VACUUM CHAMBER, MAINTAINING SAIDAUXILIARY ELECTRODE IN SAID VACUUM CHAMBER ACTIVE TO ABSORB GASESENTERING SAID VACUUM CHAMBER THROUGH SAID ORIFICE, AND MAINTAINING SAIDVACUUM PUMP ACTIVE TO CREATE A PARTIAL VACUUM PRESSURE IN SAID VACUUMCHAMBER.